WEBVTT 00:00:00.500 --> 00:00:03.975 Welcome to GCSE Edexcel Science revision. 00:00:04.125 --> 00:00:06.797 Unit C C 1: States of matter. 00:00:09.467 --> 00:00:14.991 Matter can be modelled as particles: atoms, molecules or ions depending on the substance. 00:00:15.141 --> 00:00:19.262 Drawn circles are a model, not a literal picture of every particle. 00:00:21.933 --> 00:00:26.499 In a solid, particles are close together and vibrate around fixed positions. 00:00:26.649 --> 00:00:30.071 In a crystalline solid they form a regular arrangement. 00:00:30.221 --> 00:00:32.525 A solid has a fixed shape and volume. 00:00:35.200 --> 00:00:39.436 In a liquid, particles are close together but can move past one another. 00:00:39.586 --> 00:00:45.047 A liquid flows and takes the shape of its container, while keeping approximately the same volume. 00:00:47.700 --> 00:00:52.606 In a gas, particles are much farther apart and move rapidly in random directions. 00:00:52.756 --> 00:00:57.066 A gas expands to fill its container and has no fixed shape or volume. 00:00:57.216 --> 00:01:00.307 Same particles; different arrangement and spacing. 00:01:00.457 --> 00:01:02.675 A simplified model, not to scale. 00:01:07.333 --> 00:01:12.357 Solids and liquids are difficult to compress because particles are already close together. 00:01:12.507 --> 00:01:17.150 Gases are readily compressed because there is much empty space between particles. 00:01:19.800 --> 00:01:26.071 The particles themselves do not expand, melt or change identity during a physical change of state. 00:01:26.221 --> 00:01:29.854 Their arrangement, separation, movement and energy change. 00:01:32.533 --> 00:01:34.517 Particles attract one another. 00:01:34.667 --> 00:01:38.067 The type and strength of attraction depend on the substance. 00:01:38.217 --> 00:01:46.980 For example, an ionic solid has strong attractions between ions, while a molecular solid has attractions between molecules (intermolecular forces). 00:01:49.633 --> 00:01:58.453 Heating a substance within one state generally increases particle kinetic energy: particles move faster, or vibrate more strongly in a solid. 00:02:01.133 --> 00:02:07.563 During melting or boiling, transferred energy helps overcome attractions and rearrange or separate particles. 00:02:07.713 --> 00:02:12.842 For a pure substance at constant pressure, temperature remains steady during the change. 00:02:15.500 --> 00:02:21.440 Melting and boiling take in energy; freezing and condensation transfer energy to the surroundings. 00:02:21.590 --> 00:02:25.204 Cooling within one state reduces average kinetic energy. 00:02:27.867 --> 00:02:36.974 Comparing states for one substance, gas particles usually have higher energy than liquid particles, which usually have higher energy than solid particles. 00:02:37.124 --> 00:02:41.659 Do not claim this comparison applies regardless of temperature or substance. 00:02:44.333 --> 00:02:48.044 Gas pressure results from particles colliding with container walls. 00:02:48.194 --> 00:02:53.589 A hotter sealed gas can exert greater pressure when volume and amount of gas are unchanged. 00:02:56.267 --> 00:03:00.800 Melting changes solid to liquid; freezing changes liquid to solid. 00:03:00.950 --> 00:03:03.403 These are reverse physical processes. 00:03:06.067 --> 00:03:11.687 Boiling or evaporation changes liquid to gas; condensation changes gas to liquid. 00:03:14.367 --> 00:03:19.645 Sublimation changes a solid directly into a gas, without becoming liquid first. 00:03:19.795 --> 00:03:23.701 The reverse change, from gas directly to solid, is deposition. 00:03:23.851 --> 00:03:27.017 Each pair has a forward and reverse physical change. 00:03:31.700 --> 00:03:35.737 Melting gives particles greater freedom to move while they remain close. 00:03:35.887 --> 00:03:38.788 Boiling separates particles much more widely. 00:03:41.467 --> 00:03:48.185 Freezing restricts particles to fixed positions, while condensation brings gas particles close enough to form a liquid. 00:03:50.867 --> 00:03:53.745 A physical change does not make a new substance. 00:03:53.895 --> 00:04:01.435 Water vapour, liquid water and ice all contain H 2 O molecules; suitable cooling or heating can reverse the change. 00:04:04.100 --> 00:04:09.081 Evaporation occurs at the surface of a liquid and can occur below its boiling point. 00:04:09.231 --> 00:04:12.769 Particles with sufficient energy escape into the gas phase. 00:04:15.433 --> 00:04:20.171 Boiling happens throughout a liquid at its boiling point and forms bubbles of vapour. 00:04:20.321 --> 00:04:22.604 The boiling point depends on the pressure. 00:04:22.754 --> 00:04:25.567 Both are physical changes from liquid to gas. 00:04:30.233 --> 00:04:35.311 A puddle drying on a cool day illustrates evaporation, not necessarily boiling. 00:04:35.461 --> 00:04:42.095 Water on the outside of a cold glass usually comes from condensation of water vapour in surrounding air. 00:04:44.767 --> 00:04:50.163 Dry ice is solid carbon dioxide and sublimes at normal atmospheric pressure. 00:04:50.313 --> 00:04:55.050 Frost can form by deposition of water vapour on a sufficiently cold surface. 00:04:57.700 --> 00:05:03.993 The visible white cloud above hot water consists mainly of tiny liquid droplets formed by condensation. 00:05:04.143 --> 00:05:06.553 Water vapour itself is invisible. 00:05:09.233 --> 00:05:12.517 Different substances have different melting and boiling points. 00:05:12.667 --> 00:05:17.841 Pressure also affects boiling point; always use the conditions supplied by the question. 00:05:20.500 --> 00:05:23.167 At a temperature below its melting point, 00:05:23.317 --> 00:05:25.023 a substance is solid; 00:05:25.173 --> 00:05:28.203 between its melting and boiling points it is liquid; 00:05:28.353 --> 00:05:30.571 above its boiling point it is gas, 00:05:30.721 --> 00:05:34.360 assuming the stated pressure and no chemical decomposition. 00:05:37.033 --> 00:05:42.375 For a substance melting at minus 10 degrees Celsius and boiling at 80 degrees Celsius, 00:05:42.525 --> 00:05:45.320 it is solid at minus 20 degrees Celsius, 00:05:45.470 --> 00:05:50.131 liquid at 25 degrees Celsius and gas at 100 degrees Celsius. 00:05:50.281 --> 00:05:54.699 At a transition temperature, two states can coexist during the change. 00:05:59.367 --> 00:06:05.072 At exactly a melting or boiling point, two states can coexist while the change is taking place. 00:06:05.222 --> 00:06:09.815 The temperature alone does not tell you that the whole sample has completed the change. 00:06:12.467 --> 00:06:20.071 Water at ordinary atmospheric pressure melts at about 0 degrees Celsius and boils at about 100 degrees Celsius. 00:06:20.221 --> 00:06:23.653 These familiar values are not the values for every substance. 00:06:26.333 --> 00:06:36.199 On a heating curve, sloping sections show temperature rising within a state; flat sections show melting and boiling of a pure substance under suitable constant conditions. 00:06:36.349 --> 00:06:41.191 Flat sections correspond to changes of state, rather than no energy input. 00:06:45.867 --> 00:06:54.378 Take care with negative temperatures: minus 20 degrees Celsius is colder than minus 10 degrees Celsius because it is further below zero. 00:06:57.033 --> 00:07:01.843 A useful state diagram should show spacing and arrangement as well as motion. 00:07:01.993 --> 00:07:09.095 Keep the same number and identity of particles when comparing one closed sample before and after a physical change. 00:07:11.767 --> 00:07:20.021 A simple sphere model cannot show all internal structure, real particle sizes, the full range of forces or three-dimensional motion. 00:07:20.171 --> 00:07:24.297 Arrows and labels communicate information the circles alone cannot. 00:07:26.967 --> 00:07:32.278 Gas particles move randomly between collisions; they do not all drift permanently upwards. 00:07:32.428 --> 00:07:39.274 Diffusion produces net movement from high to low concentration even though individual movements remain random. 00:07:41.933 --> 00:07:46.349 When gases mix in a closed container, they spread through available space. 00:07:46.499 --> 00:07:51.288 Mixing does not by itself mean a chemical reaction has formed a new substance. 00:07:53.967 --> 00:07:57.367 Use a substance’s properties to identify its state. 00:07:57.517 --> 00:08:06.646 A fixed shape suggests a solid; flowing while keeping a fixed volume suggests a liquid; easily compressing and expanding to fill a container suggests a gas. 00:08:09.300 --> 00:08:13.033 Practical heating and cooling observations need suitable equipment, 00:08:13.183 --> 00:08:15.786 eye protection and care with hot surfaces; 00:08:15.936 --> 00:08:19.793 temperature measurements and controlled pressure support interpretation. 00:08:22.467 --> 00:08:24.546 That completes States of matter. 00:08:24.696 --> 00:08:28.041 Revisit the notes and test yourself on the revision website.